Sendmail is a very widely used Mail Transport Agent . MTAs deliver mail from one machine to another. Sendmail is not a client program, but rather a behind-the-scenes daemon that moves email over networks or the Internet to its final destination. The configuration of sendmail in Red Hat Enterprise Linux was found to not reject the "localhost.localdomain" domain name for email messages that come from external hosts. This could allow remote attackers to disguise spoofed messages. A flaw was found in the way sendmail handled NUL characters in the CommonName field of X.509 certificates. An attacker able to get a carefully-crafted certificate signed by a trusted Certificate Authority could trick sendmail into accepting it by mistake, allowing the attacker to perform a man-in-the-middle attack or bypass intended client certificate authentication. Note: The CVE-2009-4565 issue only affected configurations using TLS with certificate verification and CommonName checking enabled, which is not a typical configuration. This update also fixes the following bugs: * sendmail was unable to parse files specified by the ServiceSwitchFile option which used a colon as a separator. * sendmail incorrectly returned a zero exit code when free space was low. * the sendmail manual page had a blank space between the -qG option and parameter. * the comments in the sendmail.mc file specified the wrong path to SSL certificates. * the sendmail packages did not provide the MTA capability. All users of sendmail are advised to upgrade to these updated packages, which resolve these issues.